Gas component of soils, Distribution, composition and properties...

Ground gas component

Distribution, Composition and Properties of the Gas Component of the Soil

Gases in any state affect the various properties of soils. Since the physical characteristics of gases are very different from the analogous parameters of the solid and liquid parts of the soil, the physical characteristics of the soil-their densities, thermophysical parameters (heat capacity, thermal conductivity, etc.), permeability, and also physico-mechanical properties, depend most significantly on gas content.

State of gases in soils . The gases in the pores of the soils can be in the free, adsorbed and jammed state; they can be present in water filling pores, in the form of small bubbles or in a dissolved state. The change in the physical and physical-mechanical characteristics of soils is affected by the content of both adsorbed and free and trapped gases.

Free gases are contained in the communicating pores of the soil, i.e., their amount depends on the open porosity of the soil and the degree of its water saturation Sr. show that free air can be in the ground only at a small degree of humidity - for S r 0.5 ... 0.6. With an increase in S r to 0.8 ... 0.9, the free gases are gradually trapped in the pores with capillary water and become cramped. Free gases in the soils of the aeration zone are under atmospheric pressure, and in soils separated from the atmosphere by impermeable rocks, they can be under high pressure, up to the formation of liquefied gases. The pressure in a free gas is determined by the Clapeyron-Mendeleev law.

Adsorbed gases are retained on the surface of ground particles under the influence of molecular forces of attraction. Due to these forces in the dry ground, the surface of the particles forms polymolecular gas films, the lower layers of which are under pressure of several tens or even hundreds of megapascals; the upper layers are less tightly bound to ground particles (the pressure they experience is close to atmospheric pressure). The amount of adsorbed gases in soils depends on their mineral composition, the presence of humus and other organic substances, on the dispersion and the magnitude of the soil porosity. Usually, the content of adsorbed gases in the soils of the podzolic strip varies from 2 to 7 cm per 100 g of soil, and for chernozem - within 8 ... 15 cm per 100 g of soil. With increasing dispersion of soil, the amount of adsorbed gases in it increases. For quartz fine-grained sand, the content of adsorbed gases was less than 1 cm per 100 g, i.e., several times less than its usual content in soils. The intensity of adsorption of the elements constituting the gas component on the surface of the mineral particles varies in a series: CCl & gt; N2 & gt; & Gt; H2, so the adsorbed gases differ in composition from those in the free state. When the soil is humidified, adsorbed gases are displaced by an aqueous film.

Pinched gases are formed in separate parts of the soil with simultaneous excessive moistening from below and from above. Cramped gases can occupy significant areas within the ground or only in small quantities in the finest micropores. Unlike adsorbed, the maximum amount of trapped gases is formed at the optimum soil moisture. The clamped gases can occupy up to 20 ... 25% of the pore volume in clay soils. The pressure in the bubbles of trapped air depends on the surface tension of the water on contact with the air. It exceeds the pressure in the surrounding capillary water by the capillary pressure determined by the Laplace equation: P cap = 2o/g, where o - surface tension coefficient water; r is the radius of the bubble of trapped air.

Dissolved gases are formed by dissolution in the pore solution. Depending on the composition of the gas and the solvent, it is possible to form the free solutions, which determine their different chemical aggressiveness, for example, the carbon dioxide (with the dissolution of CO2 in water), sulfuric acid, etc. According to Henry's law, the ratio of the molar volume of dissolved gas to the volume of liquid at a given temperature and pressure is a constant value, called the solubility coefficient, or the Henry coefficient. At the same time, it should be borne in mind that many gases (for example, HNCb, NH4, CIBO, SO2, etc.) can have increased solubility due to the formation and dissolution of other chemical forms. The formation of dissolved gases is significantly affected by temperature and pressure: with increasing pressure, the solubility of most gases increases, so their amount in the earth's crust increases with depth. But as the water warms up, the amount of dissolved air in it decreases.

Comparison of the amount of gases concentrated in oil, gas, coal and dispersed in rocks shows that the bulk is present in the sediment in the dispersed state [50].

The state of gases in the soils of the cryolithozone differs from similar thawed or unfrozen soils. This is due to the peculiarities and nature of the thermodynamic equilibrium of gases, unfrozen water and ice in frozen soils. Gases can dissolve in ice, although not as intensively as in water. Thus, the solubility of methane in ice is 3 orders of magnitude lower than the solubility in water [50].

The pinched gases in the pores of the soil are often under pressure created by various causes, for example, the movement of the freezing front, phase transitions in the freezing rock (crystallization pressure), capillary impregnation of the strata, an increase in hydrostatic pressure, an increase in temperature, etc. In a state of equilibrium, this pressure is equal to the pressure in the liquid phase contacting with the gas and causing the so-called ground pressure. The value of the pressure of a hole affects many physical and mechanical properties of soils, in particular, on their strength and compressibility under load. Compressing, the bubbles of trapped gases reduce their volume and, at a certain ratio of the diameter and size of the pores, can pass from the jammed state to free, which can be accompanied by a sharp breakthrough of gases from the soil pores and the release of pore pressure. A similar phenomenon can cause the destruction of earth embankments, dams, etc. [49].

Gases in soils according to the conditions and features of genesis can be natural and anthropogenic (technogenic) origin. Among natural gases , three genetic types of gases are identified: geological, atmospheric and biological origin. Natural gases can be syngenetic (i.e., formed simultaneously) with the formation of the rock) and epigenetic (i.e., received in the ground as a result of gas exchange with adjacent strata or with the atmosphere).

Gases of geological origin (endogenous and exogenous) are formed during magmatic (volcanic), metamorphic and radiogenic sedimentary processes. The composition and features of gases of the underground atmosphere in different parts of the earth's crust are not identical [50].

Volcanic gases that come with the magma from the deep interior of the Earth are associated with degassing. The main in their composition are water vapor (up to 90 ... 95%), followed by CO2, H2, SO2, H2S, HCl, HF, N2, NH3, Ar, He and organic compounds are present as impurities in them.
More than 60 inorganic and organic compounds are determined in the gases of hydrothermal sources , the latter being represented by hydrocarbons, volatile carbonyl compounds, alcohols, halocarbons.

Gases are also formed as a result of the pathogenetic transformation of organic matter. Basically it is methane and other combustible hydrocarbons. Oil fields are characterized by CH ", heavy hydrocarbon gases (TU), N and C02, and as impurities - H2S2.11 and noble gases. In gas fields, the set is the same, but the TU is usually present as an impurity. The same type of gas composition is typical for regions of coal deposits, but among the main gases, only catagenetic methane is to be named, all others form impurities.

Metamorphic gases are formed in subsequent (after catagenesis) stages of rock metamorphism up to their melting, in which so-called regeneration gases are released. The largest gasses on the surface of the Earth are usually associated with tectonic disturbance zones. In the composition of such gases there are water vapor H 2 0, and also CO2, N2, H2S and Ib; the main component is CO2, followed by the average concentration and frequency of occurrence of N2.

The radioactive gases are represented by noble gases. Helium, radon and argon-40 make up the bulk of radiogenic gases in soils. Uranium and thorium in the process of radioactive decay generate alpha particles, which are nothing else than the nuclei of the helium element.

Gases of the aeration zone are represented by CO2, H2, O2, the most important impurities are Ar (and other noble gases), CH4, H2. Predominantly airborne in origin are nitrogen, oxygen and noble gases. In the soil part of the earth's stratum of oxygen and nitrogen, as a rule, less than in the atmosphere. The ego is explained by the fact that in the soil there are processes of their absorption and the release of carbon dioxide. The total amount of water vapor in the soil does not exceed 0.001% of the weight of the soil. Gases of soils of the aeration zone are most often epigenetic, since they contain impurities coming from the atmosphere.

Biogenic gases are formed due to the vital activity in them of various organisms - from unicellular bacteria to higher plants, animals. The biogenic gases in the soils are mainly organic compounds. Natural combustible gases consist of methane (up to 98%), and also from a mixture of ethane, propane, butane, isobutane and pentane. Mine methane occurs during the transformation of organic residues into coal under the influence of high pressures and temperatures.

The intensity of methane extraction from wetlands varies widely. The amount of methane emission in the West Siberian swamps varies, according to N.M. Bazhina (2000), in the range from 0.1 to 40 mg/(mch). In addition to methane, biochemical (mainly during bacterial decomposition, less often when mineral salts are restored) produces carbonic i-az (CCl). hydrogen sulphide (H2S), hydrogen (Hg). The greatest value among gases is C0 2 , the amount of which in the soil air is from 0.2 to 2% (by volume).

Technogenic gases are the result of human economic activity. In soils in urban areas contains a wide range of organic gases. The buried waste releases into the surrounding strata sulfur-containing gaseous compounds (dimethyl sulphide, dimethyl disulphide, carbon disulfide, etc.), aromatic and unsaturated hydrocarbons, terpenes, alcohols and carbonyl compounds, and in the largest quantities - methane. The most dangerous are long-lived gaseous soil contaminants such as dioxins, which are ecotoxicants.

By the chemical composition of the predominant component, all the gases in the ground are divided into basic groups: hydrocarbon, nitrogen, carbon dioxide. Pure gases in soils are practically never occur: because of the ability of gases to mix easily with each other, they are most often in gas mixtures of gas mixtures of complex composition [50].

In the mining industry, the amount of gases contained in the soils determines the gas content of the mine workings, which means the volume of gas entering the gas production per unit time (absolute gas production of the mine) or the volume of the released gas, or the volume of developed rocks, coal or ore (relative gas content). With the increased pressure of gases in rocks, so-called gas dynamic phenomena are associated. These include the fast-flowing destruction of gas-bearing strata of coal, ores and rocks in the bottomhole parts of preparatory and cleaning workings, accompanied by increased gas evolution and movement or the release of the destroyed masses of rocks. In addition, the compressibility of the pore gas trapped can give the soil a kind of damping properties when the external pressure acts on the ground. With increasing pressure, the soil will shrink and decrease in volume, and when the pressure is released, the soil will expand, restoring its volume. The compressibility of trapped gases in soils can also lead to a long settling of structures.

Technogenic contamination by organic substances of the underground space in combination with the impact of buried marsh sediments contributes to the significant activation of microbiological activity of individual physiological groups of microorganisms or microbiota as a whole. In addition, an important condition for the viability of microbiota in the underground space of the city is the stagnant hydrodynamic regime of the upper aquifers and the heating effect of land structures and underground communications. The vital activity of microorganisms in the soil is accompanied by the accumulation of living and dead cells of microorganisms, the products of their metabolism, among which the most active are enzymes, organic acids, and also the gases they generate. The main product of respiration of microorganisms is CO2. Quite often in the underground space of the city biogenic sulfate reduction is observed. As a result of this reaction, evolved hydrogen sulphide even in small concentrations (3 mg/l) leads to a sharp decrease in Eh of the medium. Generation of hydrogen sulphide contributes to the formation of a secondary sulfide mineral - hydrotroilite (FeS-wI-bO), which degrades the water and mechanical properties of soils. Dissolved in the mortar gas, depending on the composition, forms chemical aggressiveness. Gases СО2 and H2S are very soluble in water and increase its aggressiveness in relation to building materials of underground structures of buildings and structures. It is known that the development of the process of karst formation in carbonate rocks is sharply enhanced with an increase in the content of dissolved carbon dioxide in the pores. Oxidation of H2S promotes an increase in the sulfate ion content and a decrease in the pH of groundwater below 4, which causes corrosion of metal, concrete, and natural stones that were used to build foundations and underground parts of structures in the 17th-19th centuries. Limestone and sandstone on carbonate cement. Oxidation of methane, formed as a result of biochemical generation, is accompanied by the formation of CO2 and water, which forms carbon dioxide aggression of water in relation to concrete, mortars made on the basis of lime and hydraulic binder. [118]

Characteristics of the gas component of the soil

The total amount of free and adsorbed gases contained in a unit of mass or volume of soil under natural conditions is called the gas-bearing capacity. In accordance with this, two indicators [50] are distinguished: bulk and mass gas content.

The gas content of the soil, G , %, characterizes the relative volume of gas contained in the pores of the soil. It is numerically equal to the ratio of the volume of gas occupied in the pores of the soil to the volume of the entire soil:

where V r is the volume of gas in the pores of the soil; volume of soil. The value of r is measured in%, depends on humidity and varies from zero (in the absence of gases and full water saturation) to a value corresponding to the porosity of the soil (when the pores completely fill with air).

The mass gas content r *, cm/g, characterizes the total content of free adsorbed gases per unit mass of the soil (w) and is numerically equal to the volume of gas contained in 1 g soil under the given melting conditions):

To assess the degree of filling with gas, use air ratio or aeration G .., which is equal to the ratio of the gas volume in the pores (K,) to the total pore volume (V n ) of the soil:

If in pores other than air there is water, then the coefficient of aeration is calculated through the degree of soil moisture according to the formula G r = 1 - S r . Measures G r in fractions of a unit and varies from 0 (in the absence of gas in the pores) to 1 (with full pore saturation with gas) [50].

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